Photoinduced hydrophilicity and structural evaluation of SiOx:OH/TiO2 multilayer films by DC reactive magnetron sputtering

We have examined the photoinduced hydrophilicity of SiOx:OH/TiO2 multilayer films containing a high degree of Si–OH bonding. The ultrahydrophilic properties were observed under UV irradiation of 10 µW/cm2, and a technique for preserving these changes in the dark has been successfully developed. Different thicknesses of SiOx:OH films were deposited onto photocatalytic TiO2 layers in an Ar/H2O ambient by DC reactive magnetron sputtering. The deposited structures were analyzed by secondary ion mass spectroscopy (SIMS), Fourier transform infrared spectroscopy (FTIR), transmission electron microscopy (TEM), and scanning electron microscopy (SEM), and their hydrophilic properties were evaluated by contact-angle measurement with respect to purified wafer. Cross sections of the structure of the SiOx:OH/TiO2 multilayer films show that SiOx:OH is deposited on the columnar-structured TiO2 film in a discontinuous manner. The amount of hydrogen in the SiOx:OH film depends on the H2O partial pressure, and the concentration depends heavily on the presence of Si–OH. The SiOx:OH/TiO2 multilayer films exhibit hydrophilic properties, even without any UV irradiation, and the hydrophilic properties under photoexcitation are also improved. The presence of –OH groups in the SiOx:OH films is considered to have an effect on this improvement in hydrophilicity.